A light emitting element display is conventionally known in which light emitting elements such as organic EL (electroluminescent) elements, inorganic EL elements, or light emitting diodes are arrayed in a matrix manner as optical elements, and the respective light emitting elements emit light to display an image. In particular, an active matrix driving type light emitting element display has advantages such as high luminance, high contrast, high resolution, and low power consumption. Therefore, such displays are developed in recent years, and particularly an organic EL element has attracted attention.
In some displays of this type, organic EL light emitting elements and a thin film transistor for driving this light emitting element by switching are combined in one pixel. A plurality of selection scan lines parallel to each other are formed on a transparent substrate. A plurality of signal lines perpendicular to these selection scan lines are also formed on the substrate. More specifically, two thin film transistors made of amorphous silicon (to be referred to as a-Si hereinafter) are formed in a region surrounded by the selection scan lines and signal lines, and one light emitting element is also formed in this region. That is, two transistors are formed in one pixel. The emission luminance (cd/m2) of an organic EL element is determined by the value per unit area of an electric current flowing through the element.
FIG. 11 shows an equivalent circuit diagram of one pixel in a conventional light emitting element display. As shown in FIG. 11, two transistors 103 and 104 are connected to a selection scan line 101 and signal line 102 per pixel. One and the other of the source and drain electrodes of the transistor 104 are connected to an emission voltage line 106 having a positive constant voltage and to an anode of a light emitting element 105, respectively.
In this structure, when the selection scan line 101 is selected (when the transistor 103 which is an N-channel transistor is turned on by applying a high-level voltage to the selection scan line 101), a signal voltage is applied from the signal line 102 to the gate electrode of the transistor 104 via the transistor 103. Accordingly, the transistor 104 is turned on, an electric current flows from the emission voltage line 106 to the light emitting element 105 via the transistor 104, and thus the light emitting element 105 emits light. When the selection scan line 101 is unselected, the transistor 103 is turned off, and the voltage of the gate electrode of the transistor 104 is held. An electric current flows from the light emission voltage line 106 to the light emitting element 105 via the transistor 104, and the light emitting element 105 emits light.
In the above structure, the magnitude of an electric current flowing between the drain and source of the transistor 104 is adjusted by adjusting the magnitude of the gate-source voltage of the transistor 104, i.e., the voltage of the signal line 102. That is, the magnitude of the drain-source current of the transistor 104 is adjusted by using an unsaturated gate voltage as the voltage applied to the gate of the transistor 104, thereby adjusting the magnitude of the electric current flowing in the transistor 104 and light emitting element 105. Consequently, the luminance of the light emitting element 105 is adjusted, and tone display is performed. Between selection and non-selection after that, i.e., during one frame period, the gate-source voltage of the transistor 104 is substantially held constant, so the luminance of the light emitting element 105 is also held constant. This driving method is called a voltage driving method by which the luminance tone is controlled by modulation of the output signal voltage from the signal line 102 to the transistor 103.
The channel resistances of the transistors 103 and 104 depend upon the ambient temperature and change after a long-term operation. Therefore, it is difficult to display images with a desired luminance tone for long time periods. Also, if the channel layers of the transistors 103 and 104 are made of polysilicon, the channel resistances depend upon the numbers of grain boundaries as the interfaces between adjacent crystal grains in these channel layers. This may vary the numbers of crystal grains in the channel layers of a plurality of transistors 103 and a plurality of transistors 104 formed in a single panel. Especially when the grain size is increased to obtain high mobility, the number of grain boundaries in the channel layer inevitably decreases, so even a slight difference between the numbers of grain boundaries in the channel length direction has a large effect on the channel resistance. This varies the magnitudes of the drain-source currents of the transistors 104 in the individual pixels, resulting in variations in the display characteristics of the individual pixels in a single panel. As a consequence, no accurate tone control can be performed. Accordingly, variations in the characteristics of the transistor 104 of each pixel must fall within a range required to control the tone of each pixel. However, as the resolution of an EL element increases, it becomes more difficult to make the characteristics of the transistors 104 of the individual pixels uniform.
As described above, in some active matrix driving EL elements, a plurality of transistors are combined as active elements formed in each pixel. In some cases, a p-channel transistor and n-channel transistor are combined. When the characteristics of carriers are taken into consideration, a polysilicon transistor functions as a p-type transistor. When an amorphous silicon transistor is used, however, good physical properties with which the transistor functions well cannot be obtained. This makes it impossible to apply amorphous silicon transistors which can be fabricated at a relatively low cost.
Some of the active matrix EL display devices as described above are not voltage driven. In some of these display devices, an active element is made up of four or more transistors in one pixel. If these transistors are formed on a substrate, the upper surface is made uneven by the thicknesses of these transistors. Therefore, an organic EL layer is desirably formed on a flat portion other than the transistor formation region. In this case, no light is emitted in this transistor formation region, so a non-light-emitting portion is inevitably formed in the pixel. When one pixel emits light with a predetermined tone luminance, the brightness can be roughly set by (emission luminance per unit area)×(emission area of one pixel)×(emission time). However, when a large number of transistors are formed, the emission area of one pixel decreases. To compensate for this small emission area, the emission luminance per unit area must be increased. Unfortunately, this shortens the light emission life because the organic EL layer is applied with a higher voltage and current. In addition, when the number of transistors in one pixel increases, the fabrication yield lowers exponentially.
Also, if too many transistors are connected in series with an EL element in a pixel, the voltage dividing ratio of these transistors rises. As a consequence, the power consumption is high.
Accordingly, one advantage of the present invention is that pixels stably display images with desired luminance in a display panel.
Another advantage of the present invention is that the display area per pixel of a display panel is increased.